1. Field of the Invention
The present invention relates to a technique for establishing synchronization between devices via a network and monitoring whether or not the synchronization being established is maintained. The present application claims priority from Japanese Patent Application No. 2010-48770, filed Mar. 5, 2010, and the disclosures described in this application are incorporated herein by reference in the designation states that accept such incorporation by reference.
2. Description of the Related Art
According to development of optical transmission technique in recent years, a signal speed increases from day to day, and communications at the signal speed of 40 Gbit/s (gigabit per second) , 100 Gbit/s, or the like, are proceeding toward practical utilization these days. With the increase of the signal speed, transmittable and receivable data volume is increased. However, when the signal speed becomes 40 Gbit/s or higher, influences such as chromatic dispersion and self-phase modulation become conspicuous, resulting in an increase of signal errors. In view of such problems, an error rate is tried to be improved according to an error correction technique, referred to as RS-FEC (Reed-Solomon Forward Error Correction) or E-FEC (Enhanced Forward Error Correction).
In the meantime, a frame contains a synchronization pattern that is used for establishing synchronization, and it is defined as a standard that this synchronization pattern is not subjected to the error correction. Therefore, after the synchronization is established, even when signals are deteriorated to a degree that the FEC is able to restore proper data, as for the data targeted for transmission, it is not possible to correct the error in the synchronization pattern, and accordingly, it may be wrongly determined as out of synchronization.
Furthermore, it is necessary to prevent unstable synchronization, such as erroneously recognizing a noise as a synchronization pattern, leading to synchronization with the noise, or even after the synchronization is established, instantaneous noise may cause a situation regarded as out of synchronization. Therefore, in general, a following method is employed: when a synchronization pattern is successively detected in serial frames, the number of which is equal to or more than a predetermined number, a transition to synchronous state takes place in the device; and when the synchronization pattern successively fails to be detected in serial frames, the number of which is equal to or more than the predetermined number, a transition to out-of-synchronization (HUNT) state takes place in the device.
The number of times the synchronization pattern is detected successively in order to shift the device from out-of-synchronization state to the synchronous state is referred to as the number of in-frame (s), and the number of times successively failing to detect the synchronization pattern in order to shift the device from the synchronous state to the out-of-synchronization state is referred to as the number of out-frame (s).
In the method as described above, when it is assumed that the number of out-frames is M2, the frame synchronization pattern length is x, the probability of mismatch of frame synchronization pattern is r, the transmission speed is S, the frame length is L, and the error rate of the transmission path is E, the misframe time Tm, from establishing synchronization until returning to the HUNT state as a result of failing to successively detect the synchronization pattern, is calculated according to the following formulas (1) and (2), for instance.
                    r        =                  1          -                                    (                              1                -                ɛ                            )                        x                                              (        1        )                                          T          m                =                                            1              -                              r                                  M                  2                                                                                    (                                  1                  -                  r                                )                            ×                              r                                  M                  2                                                              ×                      L            S                                              (        2        )            
According to the above formulas, when it is assumed that the synchronous pattern length x is 32 bits, the number of out-frames M2 is 2, and the error rate of transmission path ε is 10−4, the misframe time Tm is approximately 11.9 years in the case of STM-1 (Synchronous Transfer Mode-1). In the case of STM-64 (Synchronous Transfer Mode-64), the frame time is identical to that of STM-1 and the probability of becoming out-of-synchronization is approximately 11.9 years that is equal to the case of STM-1, even though the signal speed increases.
On the other hand, in the case of 40 G (OTU3:Optical Channel Transport Unit 3) in an OTN (Optical Transport Network), it is calculated that the out-of-synchronization occurs after a lapse of approximately 106 days. Furthermore, in the case of 100 G (OTU4:Optical Channel Transport Unit 4), since the bit number within one frame is equal to the bit number in the case of 40 G (OTU3), the frame time becomes different and it is calculated that the out-of-synchronization occurs after a lapse of approximately 41 days only. In general, it is desirable that the misframe time Tm is longer than the life of equipment.
Japanese Unexamined Patent Application Publication No. 9-18466 (hereinafter, referred to as “patent document 1”) discloses a method which subjects a synchronization pattern to error correction, and even when some errors occur in the synchronization pattern, it is possible to maintain synchronization.